1. Field of the Invention
This invention relates to liquid crystal cells, and more particularly, to electrodes for use in liquid crystal cells.
2. Discussion of the Presently Available Technology
Electrodes for liquid crystal cells are presently prepared by depositing a conductive film or coating, usually indium tin oxide (xe2x80x9cITOxe2x80x9d) film onto a surface of a pair of substrates, e.g. two pieces of glass. A liquefied polyimide is spin-coated onto the ITO surface and heat-treated to provide a polyimide coating on the ITO coating. Thereafter the surface of the polyimide layer of each of the electrodes is rubbed in the same direction with a felt cloth. Although the rubbing mechanism is not completely understood, the rubbing of the surface of the polyimide layers with a felt cloth is practiced to orient the longitudinal axis of liquid crystals in a liquid mixture contained between the electrodes in a preferred orientation. More particularly, when the liquid crystals are aligned parallel to the substrate surface and unidirectional with respect to each other there is homogeneous alignment. When the liquid crystals are aligned perpendicular to the substrate surface there is homeotropic alignment. A discussion regarding the polyimide layers is found in the letter entitled RUBBED POLYIMIDE FILMS STUDIED BY SCANNING FORCE MICROSCOPY by Y. B. Kinn, H. Olin, S. Y. Park, J. W. Choi, L. Komito, M. Matusyzyh and S. T. Lngewell published in Applied Physics Letter 66(17), Apr. 24, 1995.
Liquid crystal cells are presently fabricated by suspending fiber glass and/or polymer beads in an alcohol and spin-coating the alcohol-spacer mixture onto one of the electrodes. The electrodes are mounted with the polyimide surfaces facing one another with the direction of rubbing opposite to one another. The electrodes are maintained in spaced relationship by the spacers. One pair of opposite sides of the electrodes is sealed to provide a compartment between the rubbed surfaces of the polyimide layers. The compartment is heated to a temperature above the isotropic temperature of the liquid crystal e.g. about 90xc2x0 C. and is capillary filled with a heated mixture containing a nematic liquid crystal which may also include a chiral nematic liquid crystal or a chiral component (xe2x80x9cliquid mixturexe2x80x9d). The liquid mixture may also include a dichroic dye which provides a guest-host liquid crystal cell. The liquid mixture is preferably heated above the isotropic temperature of the liquid crystal mixture. The article entitled xe2x80x9cA POSITIVE CONTRAST GUEST-HOST DISPLAY USING A LIQUID CRYSTAL OF NEGATIVE DIELECTRIC ANISOTROPYxe2x80x9d by F. Gharadjedaghi appearing in the 1981 Vol. 68 of the Mol. Cryst. Liq. Cryst. on pages 127≅135 discusses liquid mixtures. After the compartment is capillary filled with the liquid mixture, the remaining open sides are sealed.
An AC voltage applied across the two electrode changes the position of the longitudinal axis of the liquid crystals in the liquid mixture with respect to the electrode surfaces. For example, applying an AC voltage across the electrodes orients the longitudinal axis of the liquid crystals e.g. with liquid crystals having a positive dielectric anisotropy the applied voltage will align the long axis of the liquid crystals parallel to the applied field and with liquid crystals having a negative dielectric anisotropy the applied field will align the long axis of the liquid crystal perpendicular and/or away from the applied field. The dichroic dyes can have positive or negative dichroic ratios. Dichroic dyes having a positive dichroic ratio absorb more light along the long axis of the molecule and vice versa. In the case of a guest-host nematic liquid crystal cell, applying an AC voltage across the electrodes orients the liquid crystals (positive dielectric anisotropy) parallel to the applied field, the dichroic dye (positive dichroic ratio) follows the orientation of the liquid crystal causing the cell to go from a darkened state (off state) to a bleached state (on state). When the voltage is turned off, the liquid crystals and the dichroic dyes in the liquid crystal mixture are reoriented parallel to the polyimide coating making the liquid crystal cell less transparent i.e. darker. U.S. Pat. No. 5,477,358 discusses the manufacture and operation of a liquid crystal cell.
As can be appreciated by those skilled in the art of fabricating liquid crystal cells, it would be advantageous to provide a liquid crystal cell that does not have to use the polyimide layers thereby eliminating the deposition, heat treatment and rubbing of the polyimide layer.
This invention relates to improved liquid crystal cells and more particularly, to improved electrodes for liquid crystal cells. The liquid crystal cell that is improved by the invention is of the type having a pair of electrodes spaced from one another with the edges of the electrodes sealed to provide a sealed compartment therebetween for containing a liquid mixture. The electrodes that are improved by the invention are of the type having a conductive coating e.g. an ITO coating on a glass substrate and a polyimide layer on the ITO coating. The polyimide layer has a unidirectional rubbed surface with the rubbed surface of the polyimide layers facing one another with the direction of the rubbing opposite to one another. Although not limited to the invention, the liquid mixture as used herein and in the claims may include a nematic liquid crystal and dichroic dye and may further include a chiral liquid crystal component, a chiral component, thermochromic materials and/or photochromic materials of the type including pyrons, oxazines, fulgides and fulgimides. Other photochromic materials which may be used in the practice of the invention are disclosed in U.S. Provisional Application entitled ELECTRO-OPTICAL DEVICE AND VARIABLE TRANSPARENT ARTICLE WITH SUCH DEVICE in the name of C. B. Greenberg. Thermochromic materials which may be used in the practice of the invention are those which exhibit changes in physical properties such as absorption, reflectance and refractive index as the result of temperature changes, as disclosed in U.S. Pat. Nos. 4,028,118 and 4,732,810. Without limiting the invention, dichroic dyes are useful alone in a mixture or in a mixture with photochromic materials as optically active materials mostly because of their ability to absorb light of a particular polarization when they are molecularly aligned within a liquid crystal material. Dichroic dyes that may be used in the practice of the invention, but not limiting thereto, include azo dyes and anthraquinone dyes. Other suitable dichroic dyes for use in the present invention include Congo Red (sodium diphenyl-bis-alpha-naphthylamine sulfonate), methylene blue, stilbene dye (Color Index (CI)=620), and 1,1xe2x80x2-diethyl-2,2xe2x80x2-cyanine chloride (CI=374 (orange) or CI=518 (blue)). The properties of these dyes, and methods of making them, are described in E. H. Land, Colloid Chemistry (1946). These dyes have noticeable dichroism in polyvinyl alcohol and a lesser dichroism in cellulose. Other suitable dyes include those listed with their properties and the methods of making them, discussed in the Kirk Othmer Encyclopedia of Chemical Technology, Vol. 8, pp. 652-661 (4th Ed. 1993), and in the references cited therein.
The improved electrodes of the invention eliminate the need for the polyimide layer. More particularly, the electrodes of the invention have a conductive material deposited on a substrate, the conductive material has the properties of electrical conductivity and orientation and/or alignment of the liquid crystals of the liquid mixture in the compartment. Conductive materials that may be used in the practice of the invention include, but are not limited to, pyrolytically deposited conductive metal oxides e.g. but not limiting to the invention, tin oxide having fluorine, antimony and mixtures thereof of the type disclosed in U.S. Pat. Nos. 4,584,206; 4,900,100 and 5,356,718, and in U.S. patent application Ser. No. 09/521,845 filed on Mar. 9, 2000, in the names of Janos Szanyi et al. and entitled xe2x80x9cMethods of Making Low Haze Coatings, and the Coatings and Coated Articles Made Therebyxe2x80x9d (discloses Solarban 55 coated glass). The disclosures are hereby incorporated by reference.
The invention further relates to the method of making the improved liquid crystal cell. The method includes the step of providing a substrate having a conductive material, e.g. pyrolytically depositing a tin oxide doped with fluorine and/or antimony. The surface is wiped clean using a cloth saturated with an alcohol solvent. If the wiping is random, the transmitted color of the liquid crystal cell in the darkened state is uniform as viewed by the unaided eyes and shows the random wiping pattern when viewed through a linear polarizer. If the wiping is unidirectional, the transmitted color of the liquid crystal cell in the darkened state is uniform when viewed with the unaided eyes and when viewed through a linear polarizer. A liquid crystal cell is constructed by mounting the electrodes in spaced relation and filling the space with a liquid mixture. The electrodes are seated to provide a sealed compartment containing the liquid mixture and having the electrodes in contact with the liquid mixture. As can be appreciated, the cell may be made of two substrates each having a conductive material or two substrates, only one of which has two interdigital electrodes (in-place switchingxe2x80x94{IPSxe2x80x9d)).
The liquid crystal cells made in accordance to the invention may be used as flat panel displays; air, space, land, above water and below water vehicle transparencies; residential and commercial building transparencies, transparencies for containers and lenses e.g. for eyewear. As can be appreciated, when using the electrode of the invention for eyewear, it is preferred that the electrodes are wiped unidirectionally to go from a darkened polarized state to a bleached non-polarized state.